While the Knicks have cracked down recently on hecklers and holders of opinionated signs inside Madison Square Garden, they probably cannot influence a demonstration outside the building that will urge the dismissal of Isiah Thomas, the team’s coach and president.

The rally, organized by a Long Island dentist who said he has been a Knicks fan for more than a half-century, will begin at 11 a.m. Wednesday on the Seventh Avenue side of the building and will include a pink slip that is eight feet tall and four feet wide.

The professionally printed message on the pink sign will urge James L. Dolan, the Garden’s chairman, to fire Thomas, whose team is 7-17 going into Wednesday night’s game with Cleveland. Protest organizers said two people will hold up the sign in a manner that will allow supporters to sign it.

The dentist, Dr. Art G. Nathan of Freeport, N.Y., said the idea of having a protest began a few days ago and crystallized after the Knicks lost, 119-92, on Monday night to Indiana. Late in that game, a fan with a “Fire Isiah” sign was peacefully moved out of a seat behind the team bench

Last week, during a Knicks loss to Seattle, a man a few rows behind Thomas said he was handed a printed warning from a security guard because he was heckling Thomas, although he was not using profanity.

The fan ordered from his seat Monday for wielding the “Fire Isiah” sign identified himself Tuesday as Jason Silverstein, a real estate agent who lives in Manhattan and said he goes to every home game and sits near the court, but not always in the same seat.

Silverstein, 23, said he played high school basketball in New Jersey and sponsors a summer basketball team at Rucker Park in Harlem.

“The guy is killing our team,” Silverstein said of Thomas. “How many 25-point beatings can we take?”

Silverstein said he carried some blank posterboard into the Garden on Monday night, intending to make a sign with a dark marker. Security guards confiscated it at halftime, even though nothing was written on it, he said.

Near the end of the game, Silverstein said, he made an impromptu sign by writing on a disposable food tray. After he held it up for about 45 seconds, he said, security guards said: “Get up. You’re going. You’re out.” Silverstein said he cooperated because, “I didn’t want to cause a ruckus.”

For Wednesday’s rally, in addition to the pink slip, Nathan said his group will carry picket signs with messages like “Save the Knicks,” “Dump Isiah,” “Give Him the Pink Slip” and “Restore Knicks Pride.”

“I’m getting disgusted,” said Nathan, who is 67. “And James Dolan is not going to do a darn thing. We hope we have thousands of people. It’ll be a beautiful sight to see.”

The group has told the Police Department that the demonstration will probably include about 15 people, said Officer Martin Brown, a police spokesman, and last for about 30 minutes.

Nathan said he had followed the team since “the McGuire boys at the old Garden,” and added that his group would conduct its demonstration behind a police barricade.

“They are perfectly within their rights,” said Brown, who added that a permit would be needed only for a large number of people or use of a sound system.

Jonathan Supranowitz, a Knicks spokesman, said the team had no comment about the proposed demonstration.

Supranowitz had more to say about Silverstein’s “Fire Isiah” sign. Supranowitz said Silverstein was in a section where seats sell for $330 and had a valid ticket. His sign was removed, Supranowitz said, because it might have blocked the view of other spectators.

Supranowitz said security guards escorted the man to the corridor behind the seats and watched him walk away, but did not eject him. Supranowitz said the heckling and sign-waving recently directed against Thomas are “not against the law, it’s just against our policy.”

The policy, which is detailed on the Web site thegarden.com, said in part that “signs may not be derogatory in any manner” and “they cannot block the view of other guests.” Violators, the policy said, can have their sign confiscated and may face ejection.

Supranowitz said the Knicks would not reveal how many fans have had signs confiscated recently, how many have been warned about heckling, or how many have been ejected for violating either policy.

The heckler who was warned last week, Michael Katz, an accountant from Westchester County, was not ejected and voluntarily moved to another seat.

The card he was handed read, in part, “You are being issued a warning that the comments, gestures and/or behaviors that you have directed at players, coaches, game officials and/or other spectators constitute excessive verbal abuse.”

Of course, in addition to the actions and words of individuals, fans as a group have chanted “Fire Isiah” during recent defeats.